1. Field of the Invention
The present invention relates to a six-bar linkage positioning mechanism and more particularly, to such a six-bar linkage positioning mechanism practical for use in a clean container for holding a plurality of workpieces which vertically spaced and horizontally arranged in position without producing friction.
2. Description of Related Art
In a semiconductor foundry, wafer cassettes are used to hold semiconductor wafers in horizontal at different elevations. A wafer cassette for this purpose has bearing flanges, defining a plurality of horizontal insertion slots at different elevations for accommodating semiconductor waters. When a wafer cassette is fully loaded with semiconductor wafers, the wafer cassette is then mounted on the base of a clean container, and then a cover shell is covered on the base to protect storage wafers against contamination.
Conventionally, a wafer cassette is simply received in a wafer container after having been loaded with wafers. However, because the inside space of a conventional wafer container is much greater than a wafer cassette, storage wafers may slip inside a wafer container during its transportation. Therefore, a wafer container is provided with a retainer on the inside for holding storage wafers in position. During installation of a wafer container, a slide is stopped at the base of the wafer container to move a retainer forward, thereby causing the retainer to push storage wafers into position.
Further, in mechanisms, the so-called “degree of freedom” means the extent to which a mechanical element is freely movable without constraint. If the degree of freedom is one, it means a constrained motion mechanism, i.e., all parts of the mechanism will move in a specific direction when the mechanism is forced by an external force. If the degree of freedom is two or over two, it means a free motion mechanism, i.e., the parts of the mechanism will move in two unexpected directions when forced by an external force. In this case, it cannot accurately control the motion path and thereby unable to estimate the safeness of the motion mechanism.
In the linkage of a planar mechanisms, the degree of freedom is calculated by means of Gruebler's formula, i.e., F=3(N−1)−2P1, wherein F is the degree of freedom of the linkage; N is the total number of links; P1 is the total number of the motion pair of the lower pair, such as rotation pair or sliding pair.
FIG. 1 and FIG. 2 show a wafer container constructed according to the prior art. As illustrated, the wafer container 9 comprises a top cover shell 91 and a base 92. The top cover shell 91 is covered on the base 92, having a retaining mechanism 93 provided on the inside for pushing wafers 941 in the cassette 94 carried on the base 92.
As illustrated in FIG. 3, the retaining mechanism 93 is a five-bar crank and slider mechanism comprising a fixed rod member 95 (which is a locating member fixedly fastened to the inside wall of the top cover shell 91), a top link 96, a front link 97 (retainer for pushing wafers 941 into position and then holding them in position), a bottom link 98, and a sliding rod 99. This five-bar crank and slider mechanism has total four rotation pairs 951, 961, 971 and 981 and one sliding pair 991. According to the aforesaid Gruebler's formula F=3(5−1)−2×5=2, i.e., this mechanism is an unconstrained motion mechanism having two degree of freedom.
Referring to FIG. 4˜FIG. 6 and FIG. 1 and FIG. 2 again, when the retaining mechanism 93 started to move, the front link 97 is tilted in one direction to push the wafers 941 upwards due to the reason of degree of freedom, as shown in FIG. 5. At this time, the wafers 941 do not receive applied force evenly, thereby causing friction between the wafers 941 and the front link 97. Friction between the wafers 941 and the front link 97 may produce particles of dust to contaminate the wafers 941.
FIG. 7 shows another linking motion of the aforesaid prior art design. Because the aforesaid five-bar crank and slider mechanism is an unconstrained motion mechanism having two degree of freedom, the front link 97 may be tilted in another direction to push the wafers 941 downwards. This motion causes the same problem.
Therefore, it is desirable to provide a six-bar linkage positioning mechanism that eliminates the aforesaid problem.